Project Summary/Abstract Carotid webs (CaW) are intraluminal shelf-like projections into the internal carotid artery (ICA) bulb that have been reported to occur in up to 37% of patients with cryptogenic stroke. The presence of the CaW in the flow field causes formation of a large flow recirculation zone distal to the web that creates pro-thrombogenic conditions such as high particle residence time and near-wall flow stasis conditions. The goal of this project is to understand the hemodynamic mechanisms of CaW-related thrombus formation and subsequent stroke risk in order to provide patient-specific prognostication for assessing treatment options. To assess the project goal we will quantify carotid artery hemodynamics using 4D flow MRI in patients with CaW and in healthy, age-matched control subjects with no carotid artery disease. We hypothesize that patients with CaW will demonstrate pro-thrombogenic hemodynamic metrics compared to normal subjects including; high particle residence time, high fluid shear gradients near the web, development of flow vortices distal to the web, and low flow (stasis) regions in the recirculation zone. We will also conduct a brain MRI to correlate the pro- thrombogenic flow patterns to evidence of previous stroke or evidence of small vessel cerebrovascular disease. Successful completion of the project will provide in-vivo evidence of the mechanism of cryptogenic stroke in patients with CaW and lay the groundwork for 4D flow MRI as a tool for management of relative stroke risk in CaW patients.